This project is based on preliminary studies of chromosome complements in the genus Lemur using light microscopic analysis of G-banded mitotic chromosomes and electron microscopic analysis of synaptonemal complexes (SC's) in whole mount spreads of spermatocytes. From homologies between certain acrocentric chromosomes of individuals with high diploid numbers and the arms of metacentrics from individuals with lower diploid numbers, it was concluded that the latter evolved from the former by "centric fusion", and that such Robertsonian rearrangements have played a principal role in chromosome evolution in the Lemurs. The unique approach of G-banding correlated with electron microscopy of meiotic pairing, particularly of metacentric-acrocentric heterozygotes, provides substantial evidence with which to support the proposition and remove it from the realm of speculation. This hypothesis will be tested and extended by 1) further G-band analysis of additional intra- and interspecific chromosomal variants and hybrids, 2) examination of the arrangement and behavior at meiosis of various hybrids heterozygous for the polymorphic chromosomes by electron microscopy of the SC correlated with spreads for light microscopy, and 3) electron microscopy of G-banded mitotic chromosomes to identify and characterize fully all chromosomes, including small acrocentrics. Particular attention will be paid to electron microscopy of SC's in interspecific hybrids (L. macaco (2N equals 44) x L. fulvus rufus (2N equals 60), and L. macaco x L. f. collaris (2N equals 51)), C-banding of mitotic chromosomes, and breeding experiments to determine the fertility of the hybrids. It is expected further that these studies will contribute new evidence 1) for assessing phyletic relationships within the Lemurs, and 2) in support of the possibility that the kinds of changes observed may be important in the genetic isolation of sibling species.